Group antennas for microwaves comprising a desired number of parallel cavity waveguides are known. The cavity waveguides are thereby placed adjacent to each other and on the front sides of the cavity waveguides, a great number of short slots are arranged one after the other, through which microwave energy is emitted to and/or is taken up from the surroundings. The slots are normally evenly spaced along the cavity waveguides. The cavity waveguides may according to a suitable point of view be looked upon as resonance chambers, from which microwaves may be emitted through said slots.
In U.S. Pat. No. 5,028,891 an antenna of this type is described, in which the cavity waveguides, which preferably are comprised of ridge waveguides are fed via a number of adaptation chambers in which a central conductor is arranged in a substrate. Each adaptation chamber is fed by a coaxial cable and is arranged in direct communication with one of the cavity waveguides in such a way that one of the walls of the same is formed by one of the walls of the cavity waveguide. In this wall a preferably H-shaped slot is arranged through which microwaves are transmitted from the adaptation chamber to the cavity waveguide.
The construction described in U.S. Pat. No. 5,028,891 having adaptation chambers is, however, expensive and relatively complex. High demands are for instance made on the adaptation chamber fitting tightly against the cavity waveguide. Each adaptation chamber for the group antenna needs individual mounting and adjustment with small tolerances.
The shown construction also demands relatively much space depthwise, which presents a substantial drawback in antenna constructions where the available space often constitutes a limiting factor. This fact is accentuated in mobile applications.
Power transmission of microwaves between different transmission conductor devices using slots is also known in other contexts. U.S. Pat. No. 5,539,361 shows a transition section between a cavity waveguide and a microstrip conductor. The cavity waveguide exhibits a continuously tapering form up to an aperture around which the cavity waveguide preferably is tightly applied to an earth plane on the microstrip card. A slot is arranged in the earth plane opposite this aperture. This slot is the same size or smaller than the aperture in the cavity waveguide. The cavity waveguide is adapted to transmit microwaves in its longitudinal direction up to the aperture. As the slot is small in comparison to the cross-section of the cavity waveguide reflections tend to arise. To try to counteract this effect the cavity waveguide exhibits a slowly tapering cross-section.
Also for the construction described in this document it is true that much care is required to accomplish a tight transition in order to avoid power losses. Further, this construction is sensitive to a possible displacement of the aperture in relation to the slot in the earth plane. This is especially so, when the aperture is approximately as big as the slot. If the slot is smaller than the aperture, problems arise with reflections giving less efficiency.